THE ANCESTRY OF THE VERTEBRATES 89 



to the Chordates, the occurrence of this larva is a 

 severe blow to the Annelid theory of Chordate de- 

 scent, because the Tornaria larva in all its characters 

 is the very opposite to the Trochosphere larva of the 

 Annelids (see pp. 69, 71). 



It is in fact impossible to associate the Echino- 

 derms with the Annelids, in the ancestry of the 

 Chordates, as they really possess nothing in common. 

 If we persist in upholding the Annelid theory, there 

 is nothing for it but to deny the relationship of 

 Balanoglossus to the Chordates, and this is a measure 

 which few would be confident enough to take. 



This difficulty is in reality the great crux that 

 besets the problem of the origin of the Chordata 

 from any existing Invertebrate phylum. It is not 

 from lack of resemblances between the Vertebrata 

 and Invertebrata that we are at a loss ; there is 

 much to be said for the Annelid theory, and much 

 also for the inclusion of Balanoglossus in the Chor- 

 data, but the acceptance of the one excludes the 

 other. Further than this unsatisfactory conclusion 

 we are not prepared to go, and we must consign the 

 question of the origin of the Chordata to the category 

 of unsolved morphological problems. 



There are certain other animals of obscure re- 

 lationships which are held to belong to the same 

 stock as the Echinoderms and to Balanoglossus and 

 the Chordates. 



